Solid state imagers, including charge coupled devices (CCD) and CMOS sensors, have been used in photo imaging applications. A solid state imager circuit includes a focal plane array of pixel cells, each one of the cells including either a photogate, photoconductor or a photodiode having a doped region for accumulating photo-generated charge. Microlenses are placed over imager pixel cells. A microlens is used to focus light onto the initial charge accumulation region. A single microlens may have a polymer coating, which is patterned into squares or circles provided respectively over the pixels which are then heated during manufacturing to shape and cure the microlens.
Use of microlenses significantly improves the photosensitivity of the imaging device by collecting light from a large light collecting area and focusing it on a small photosensitive area of the sensor. The ratio of the overall light collecting area to the photosensitive area of the sensor is known as the fill factor of the pixel.
The use of smaller sized microlens arrays is increasingly important in microlens optics. One reason for increased interest in reducing the size of microlenses is the increased need to reduce the size of imager devices and increase imager resolution. However, reductions in pixel sizes result in a smaller charge accumulation area in individual pixels in the array. Reduced sizes of pixels result in smaller accumulated charges which are read out and processed by signal processing circuits.
As the size of imager arrays and photosensitive regions of pixels decreases, it becomes increasingly difficult to provide a microlens capable of focusing incident light rays onto the photosensitive regions. This problem is due in part to the increased difficulty in constructing a smaller microlens that has the optimal focal characteristics for the imager device process and that optimally adjusts for optical aberrations introduced as the light passes through the various device layers. Also, it is difficult to correct possible distortions created by multiple regions above the photosensitive area, which results in increased crosstalk between adjacent pixels. “Crosstalk” can result when off-axis light strikes a microlens at an obtuse angle. The off-axis light passes through planarization regions and a color filter, misses the intended photosensitive region, and instead strikes an adjacent photosensitive region.
Microlens shaping and fabrication through heating and melting microlens materials becomes increasingly difficult as microlens structures decrease in size. Previous approaches to control microlens shaping and fabrication do not provide sufficient control to ensure optical properties such as focal characteristics or other parameters needed to provide a desired focal effect for smaller microlens designs. Consequently, imagers with smaller sized microlenses have difficulty in achieving high color fidelity and signal/noise ratios.